MicroBug Machining

Making the Phone Jack Holder / Front Foot

     Many of my key designs call for a 1/8" phone jack for electrical connections.  The most useful jacks for this purpose are the "inline" type that are designed for mounting on the end of a cable.  They do not have any provisions for chassis mounting, so a method needs to be devised.  The most reliable one that I have found requires a groove to be machined in the the instrument base (with a diameter equal to that of the jack barrel.  The jack is held by a clamp that is the other "half" of the hole.  The clamp also doubles as the front foot of the bug or paddle.  There are a number of ways to produce clean "half-holes", and one that I have worked out is shown in these pictures.


     The MicroBug, which is now undergoing revision, uses the clamping system discussed above.

      This is an older version of the key to show how the front phone jack is mounted.  It requires a 5/16" hole to hold the barrel of the jack.  (The newer versions use a jack with a diameter of 3/8", which is too large to fit into a hole drilled into the end of a 3/8" thick base)

    At the left is the blank base of the bug (0.5"x1.5"x2.5") and a piece that will become the bottom clamp for the jack.  Its present dimensions are 0.25"x1.0"x1.5", and its final dimensions will be 0.25"x0.813"x0.625".  This does result in some waste (which is necessary for the method), but the scrap pieces can be used for something else (I guess).  Above the blank pieces is a drilling template that will be used to locate holes in the base blank.  At the right is the clamp blank, with bandsawed ends.  It will be aligned for trimming in an aluminum block that fits its width and will be mounted in the vise.  My shop contains a number of such jigs for various sizes of stock, plus a variable one for odd sizes.

     At the Upper Left (UL) is the piece mounted in the milling vise.  It will have its end trimmed (UR) with a 0.022" slitting saw, which leaves a very smooth edge.  The cut is in progress at the Lower Left (LL), and the final surface is at the Lower Right (LR).  Where is can be used, the slitting saw produces much cleaner ends with less fuss than using an end mill.

      The other end has now been cut to length, and the piece now needs to be cut to width.  The UL shows a jig for horizontal mounting of the piece, which is shown in the UR view.  The cut needs to be 0.0625" plus the thickness of the saw.  This is easily achieved by using a piece of 1/16" stock to guage the position of the saw blade (another advantage over the end mill), and the LL shows the cut in progress.  At the LR is the piece with both sides trimmed off, and it is ready for further machining.

      There is now a detour while the relevant holes are made in the bug base (using the template shown earlier).  The two center holes (UL) are #43, and tapped for 4-40 threads (the outer holes are for the binding posts).  At the UR are the pieces that will now be joined together to transfer the hole locations.  A ratchet clamp (LL) holes the smaller piece exactly centered and flush with the end of the base.  This assembly is inverted and one of the threaded holes is used as a drill guide to mark the clamped piece.  There is no detectable damage to the threads as long as a vertical position is maintained.  (The damage to the operator's thumb is the result of a bit overconfidence when drilling a piece of brass hand-held rather than in a vise.)

      At the UL, the marked hole is drilled out with a #33 drill (to pass the body of a 4-40 screw.  This hole is countersunk for a flat-head screw (UR). This hole is used to screw the clamp piece into position.  After it is carefully aligned, the other hole is used to guide a #43 drill to mark its location on the clamp.  This hole is drilled out to #33 and countersunk as before.  It is then mounted in position (LR), ready for the drilling of the 3/8" hole.

      The end of the pair is coated with machinists' dye for marking the drilling location (UL).  The vertical mark is exactly centered, and the junction between the two pieces is the horizontal mark (UC).  At the UR the assembly is mounted vertically in the vise.  A laser center finder is used to locate the exact place to drill (a neat tool, but one can use a spiral-point tap or other accurate point).  A center drill (LL) begins the drilling process, exactly on the dividing line.  This small hole is use to guide a 3/8" slow-spiral drill, especially made for drilling metals like brass (LC).  A conventional bit could be used if a lot of care is taken (the operator's thumb shown above is a reminder that conventional bits will grab in brass).  At the LR is the finished hold, divided equally between the clamp piece and the base.  The extra length is used to keep the load on the drill symmetrical - if the clamp were cut to length first (5/8") and then the assembly drilled to a depth of 1", unpleasant things would happen when the drill emerged into the half-hole region.  This can be done with care, but there will be some inevitable wandering and over-sizing of the hole.

      The drilled assembly (UL) is ready for further work, and the separate parts are shown at the UR.  The phone jack fits into groove in the base (LL). Because the diameters of the phone plug is only nominally 3/8", the fit is very slightly loose.  There are several remedies for this.  A thin shim can be added to the assembly to take up the slack, or the clamp piece (after trimming to length) could be milled slightly thinner.  A third way is shown at the LR, where a hole for a setscrew is drilled and tapped 4-40.

      Now the clamp can be cut to length (0.625").  The distance is determined by using dimension-stock spacers (UL) to bring the saw to the marked line (UR).  The piece is shown in a different view at the LL, and after the cutting at the LR.  It is now ready to serve as the lower clamp for the phone jack.

    The cut clamp, and its mating grove in the base, shown with the phone jack locate in the base groove (L), and the finished assembly is at the right.  All the bug now needs is about 15 more holes drilled in the base, and the working parts for the mechanism above, and it will be done.  Still a long way to go, but this is a start.

Final Comments

     This is but one way to do this operation.  If several similar pieces are being made (for instance, two identical bugs), their bases can be mounted together and the pair drilled along the mating line.  The lower clamp part could be made from a sandwitch of 1/4" stock of the proper width and length.  Alternatively, one could use a 3/8" end mill and carefully mill out the channel.   The advantage to the method given above is that "it can't not fit", because the pieces were drilled while held in their final positions.


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